1. Technical Field
The present invention relates, generally, to a MOSFET control circuit with controlled channel voltage drop and, more particularly, to an improved circuit which provides reverse bias blocking through the use of a diode, but which utilizes a voltage-controlled MOSFET channel in lieu of the blocking diode for forward voltage conduction.
2. Background Art and Technical Problems
Control circuits are often required to deliver a pre-determined threshold voltage to an electrical load, even when the input voltage dips below the threshold voltage required to drive the load. In the case of power supply circuits, for example, it is quite common to augment the power supply with a bank of capacitors configured to discharge their stored energy to the load during the transient periods in which the input voltage source dips below the require threshold voltage, thereby insuring that adequate voltage is supplied to the load at all times. This is particularly important in applications involving health and safety, for example in the avionics industry to maintain power supply to avionics computers, flight control systems, and other on board aircraft electrical loads.
Presently known control circuitry for supplying a threshold voltage to a load notwithstanding transient reductions in the input voltage below the load threshold typically involve the use of a power converter circuit. In particular, a power supply includes a diode through which power is applied to the load, with a bank of capacitors disposed in parallel with the load. Thus, when the input power supply through the diode exceeds the threshold voltage to be applied to the load, the capacitors remain charged and the load "sees" an applied voltage which is at least as great as the threshold level required by the load. When the input voltage (supplied by a battery, generator, or the like) dips below the threshold level, such that the output voltage exceeds the input voltage, the diode blocks current flow in the reverse bias direction, and the capacitors release their energy to the load to maintain the voltage supplied to the load at or above the threshold level until the input source again reaches the load threshold. This technique for maintaining threshold voltage to the load, however, is disadvantageous in several respects.
In particular, the reverse blocking diodes employed in presently known power supplies typically exhibit a voltage drop on the order of 0.5 to 0.8 volts. At high current values, a substantial amount of power is dissipated by the diode, such that the circuit liberates a significant amount of thermal energy. In order to dissipate the heat in a controlled manner, heat sinks and associated hardware are often employed. Moreover, high operating temperatures contribute to diode failure in many applications.
Control circuits are thus needed which effectively isolate the load from the source during transient low voltage conditions at the source, but which reduce power losses and high operating temperatures associated with presently known systems.